Mobile station and communication system

ABSTRACT

A mobile station according to the present invention includes: a first measuring means ( 32 ) for measuring received power strength and propagation loss of signals transmitted from a communicating first base station; a second measuring means ( 32 ) for measuring received power strength and propagation loss of signals transmitted from a second base station neighboring the first base station; a first derivation means ( 34 ) for deriving the difference between the received power strengths from the first and second base stations, as measured by the first and second measuring means; a second derivation means ( 34 ) for deriving the difference between the propagation losses from the first and second base stations, as measured by the first and second measuring means; a comparing means ( 40 ) for comparing the values derived by the first and second derivation means ( 34 ); and a transmitting means ( 42 ) for transmitting, based on the comparison result by the comparing means ( 40 ), a handover signal to the first base station. Thereby, the mobile station that executes the handover process without fail and improves communication quality can be provided, even if a region in which upload signals can reach the communicating base station is different from another region in which download signals can be received from the communicating base station.

TECHNICAL FIELD

The present invention is related to mobile stations improved in order toprevent communications disconnection at handover.

BACKGROUND ART

In Section 14.1.2.1 of the 3GPP (3rd Generation Partnership Project)specification TS25.331V3.11.0 (2002-06), the following formula (10) isdescribed as a condition for executing a handover process. Therefore, amobile station is configured to execute the handover process when thecondition is satisfied.

FIG. 7 is a block diagram illustrating a configuration of a mobilestation. In FIG. 7, the mobile station “100” transmits signals processedby a microcomputer “120” to a base station in a communication modethrough a baseband unit “140”, a radio unit “160”, and an antenna “180”.Then, the mobile station 100 receives signals from the base stationthrough an opposite route.

The mobile station 100 receives signals through a first common channelPCIPCH (primary common pilot channel) from another base station, in anon-communication mode (called “monitor set cell” in the 3GGP Standard),that is neighboring the base station in the communication mode (called“active set cell” in the 3GGP Standard). The received signals areinputted into the microcomputer 120 through the antenna 180, the radiounit 160, and the baseband unit 140. A measurement processing unit “122”of the microcomputer 120 periodically measures received power strengthsfrom the neighboring base station and the base station in thecommunication mode, and inputs the measurement result into an event 1Ajudging unit “124”. The event 1A judging unit 124 judges whether theFormula (10) is satisfied or not based on measurement results of thereceived power strengths from the base station in the communication modeand the neighboring base station.

When the Formula (10) is satisfied, the mobile station outputs a signalrepresenting an event occurrence of the handover process to the basestation in the communication mode through the baseband unit 140, theradio unit 160, and the antenna 180. In addition, the measurementprocessing unit 122 measures received power strengths from the basestation in the communication mode as well as from the neighboring basestation in the non-communication mode. $\begin{matrix}{{{10 \times {Log}\quad{MNew}} + {CIONew}} \geq {{W \times 10 \times {{Log}\left( {1/{\sum\quad\left( {1/{Mi}} \right)}} \right)}} + {\left( {1 - W} \right) \times 10 \times {Log}\quad{Mbest}} + \left( {{R\quad 1a} - {H\quad 1{a/2}}} \right)}} & {{Formula}\quad(10)}\end{matrix}$

In addition, in the left side of the Formula (10), “MNew” is receivedpower strength from the neighboring base station as the monitor setcell, which is measured by the measurement processing unit 122, and“CIONew” is a correction value of the MNew. Moreover, “0” or “1” isassigned to “W” in the right side of the Formula (10), the first termthat is total received power strength “Mi” from one or plurality of basestations, as active set cells, in the communication mode or the secondterm that is the highest received power strength “Mbest” from the basestations is selected, and a value calculated by using the selected termis compared with the received power strength from the neighboring basestation, as the monitor set cell, in the left side of the Formula (10).Moreover, “R1a” is a reporting range for defining occurrence conditionsfor the Event 1A, and “H1a” is correction value, referred to as ahysteresis value, of the reporting range R1a.

Next, the Formula (10) will be explained according to FIG. 8. Inaddition, in order to explain the Formula (10) simply, 0 is assigned tothe W, the CIONew, and the H1a in this case. In FIG. 8, the receivedpower strength from each of the base stations is illustrated in thevertical axis, and time is illustrated in the horizontal axis. Inaddition, the received power strength is increased when the position isshifted upward along the vertical axis. In FIG. 8, the value of thesecond term in the right side is illustrated as a curve “1”, the valuein which the value of the reporting range R1a is subtracted from thevalue of the second term, is illustrated as a curve “1a”, and the valuein the left side is illustrated as a curve “2”. In FIG. 8, the Formula(10) is satisfied at a point “C”, where the value in the left sideexceeds the value in which the value of the reporting range R1a issubtracted from the value of the second term in the right side.

Next, the operations of the mobile station 100 are explained accordingto FIG. 9.

In the mobile station 100, the event 1A judging program is periodicallyexecuted. When the event 1A judging program is executed, themicrocomputer 120 firstly judges whether the Formula (10) is satisfiedor not (S101). By this judging, when the value in the left side is lowerthan the value in the right side, the event 1A judging unit 124terminates the event 1A judging process without reporting an eventoccurrence to an event transmitting unit “126”.

On the other hand, when the microcomputer 120 judges that the Formula(10) has been satisfied, and if a predefined time has passed after thejudgment (S103), the event 1A judging process is terminated afterreporting the event occurrence to the event transmitting unit 126(S105).

As described above, because soft handover process is executed in themobile station 100 when the Formula (10) is satisfied, the handoverprocess can be executed when the received power strength (value of thefirst term or the second term in the right side of the Formula (10))from the base station as the active set cell approximately balances thereceived power strength (value of the first term, in the left side ofthe Formula (10)) from the base station as the monitor set cell. Inaddition, even if the Formula (10) is satisfied, the event occurrence isnot reported to the event transmitting unit 126 unless the predefinedtime has passed, because unnecessary information must not be reported tothe base station when the Formula (10) is instantaneously satisfied.

However, a region in which download signals from the base stationreaches the mobile station may sometimes be greatly different from aregion in which upload signals from mobile station reaches the basestation.

Therefore, in the above conventional technology, a position in which thehandover process is executed may sometimes be a position in which theupload signals can not reach the base station as the active set cell.

For example, as illustrated in FIG. 10, it is assumed that a region “Ad”in which download signals from a base station “A” as an active set cellcan reach the mobile station 100 is wider than a region “Au” in whichupload signals can be transmitted from the mobile station 100 to thebase station A, and a region “Bd” in which download signals from a basestation “B” as a monitor set cell can reach the mobile station 100 isnarrower than a region “Bu” in which upload signals can be transmittedfrom the mobile station 100 to the base station B. In this case, asillustrated in FIG. 10, a position “Ps” (position where the handoverprocess is executed) in which the download signals transmitted to themobile station 100 from the base station A and B balance each other, maysometimes be a position (position outside the region Au) in which theupload signals can not reach the base station A as the active set cell.In addition, the mobile station 100 must transmit/receive informationto/from the base station as the active set cell in order to perform thehandover process.

In other words, the mobile station 100 can not transmit the informationto the base station A so that it fails in the handover process when thehandover process is executed, and communication quality may sometimes bedeteriorated.

Therefore, the present invention has been made in order to provide amobile station that executes the handover process without fail andimproves the communication quality, even if the region in which uploadsignals can reach the base station in the communication mode isdifferent from the region in which download signals can be received fromthe base station in the communication mode.

DISCLOSURE OF THE INVENTION

The present invention is related to a mobile station including: a firstmeasuring means for measuring received power strength and propagationloss of signals transmitted from a communicating first base station; asecond measuring means for measuring received power strength andpropagation loss of signals transmitted from a second base stationneighboring the first base station; a first derivation means forderiving the difference between the received power strengths from thefirst and second base stations, as measured by the first and secondmeasuring means; a second derivation means for deriving the differencebetween the propagation losses from the first and second base stations,as measured by the first and second measuring means; a comparing meansfor comparing the values derived by the first and second derivationmeans; and a transmitting means for transmitting, based on thecomparison result by the comparing means, a handover signal to the firstbase station.

Thereby, the present invention can provide a mobile station thatcertainly executes a handover process and improves the communicationquality, even if a region in which upload signals can be transmitted tothe base station in the communication mode, is different from a regionin which download signals can be received from the base station in thecommunication mode.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a mobile station according toEmbodiment 1 of the invention;

FIG. 2 is a flow chart illustrating the operation of the mobile stationaccording to Embodiment 1 of the invention;

FIG. 3 is a diagram for explaining the operation of the mobile stationaccording to Embodiment 1 of the invention;

FIG. 4 is a sequence diagram illustrating a handover process between themobile station and base stations;

FIG. 5 is a diagram for explaining the operation of the mobile stationaccording to Embodiment 1 of the invention;

FIG. 6 is a sequence diagram illustrating a communications systemaccording to Embodiment 2 of the invention;

FIG. 7 is a diagram illustrating a mobile station according to aconventional technology;

FIG. 8 is a diagram for explaining a judgment formula used in theconventional technology;

FIG. 9 is a flow chart illustrating the operation of the mobile stationaccording to the conventional technology; and

FIG. 10 is a diagram for explaining the operation of the mobile stationaccording to the conventional technology.

BEST MODE FOR CARRYING OUT THE INVENTION Embodiment 1

A configuration and operations of a mobile station related to Embodiment1 will be explained according to FIG. 1 through FIG. 5.

A mobile station “10” transmits signals processed by a microcomputer“30” to any of base stations through a baseband unit “50”, a radio unit“70”, and an antenna “90”. Moreover, the mobile station receives signalson a “PCIPCH” (primary common pilot channel) from neighboring basestations (Whether the communication mode is “active set cell” or“monitor set cell” doesn't matter.) specified by the communicating basestation. The received signals are inputted into the microcomputer 30through the antenna 90, the radio unit 70, and the baseband unit 50. Themicrocomputer 30 includes: a measurement processing unit (firstmeasuring means and second measuring means) “32” for periodicallymeasuring received power strength from the each of the base stationsaccording to input signals; a α/β derivation unit (first derivationmeans and second derivation means) “34” for calculating the followingformulas (1) and (2) based on a measurement result by the measurementprocessing unit 32 and deriving α and β values; an a storage unit “36”and a β storage unit “38” for storing each of derived values by the α/βderivation unit 34; an event 1A judging unit (comparing means) “40” forjudging whether the following formula (3) is satisfied or not by usingthe a value stored in the storage unit 36 and the β value stored in thestorage unit 38; and an event transmitting unit (transmitting means)“42” for transmitting, a signal (measurement report) for reporting anevent occurrence to the base station as the active set cell through thebaseband unit 50, the radio unit 70, and the antenna 90, based on thejudgment by the event 1A judging unit.

In addition, the received power strength measured by the measurementprocessing unit 32 may be calculated using either “RSCP” (receivedsignal code power) from any of the base stations, a ratio (power ratioof received signal code “Ec/No”) of the received power strength from anyof the base stations to the received power strength from another basestation, or electrical field intensity of the received signals.α=(−1)×(10×Log MNew+CIONew−(W×10×Log(1/Σ(1/Mi))+(1−W)×10×LogMbest+(R1a−H1a/2))  Formula (1)β=10×Log MNew+CIONew−(W×10×Log(ΣMi)+(1−W)×10×LogMbest−(R1a−H1a/2))  Formula (2)

In the above formulas, although each of symbols means the same contentas described in Section 14.1.2.1 of the 3GPP specificationTS25.331V3.11.0 (2002-06), meanings of the symbols will be explainedbelow according to the configuration of the mobile station 10.

“MNew” is a value measured by the measurement processing unit 32 of apredefined base station (a second base station neighboring a first basestation) as a monitor set cell, and the value means a propagation lossof signals transmitted from the base station as the monitor set cell inFormula (1), and a measurement result of received power strength of thesignals transmitted from the base station as the monitor set cell inFormula (2).

“CIONew” is a correction value of the “MNew”, and is reported from thebase stations. “Mi” is a value measured by the measurement processingunit 32 of another base station (the communicating first base station)as an active set cell, and the value means a propagation loss of signalstransmitted from the base station as the active set cell in Formula (1),and received power strength of the signals transmitted from the basestation as the active set cell in Formula (2).

Moreover, if a plurality of base stations exists, as the active set cellof the mobile station 10, “MBest” means a propagation loss of the basestation having the lowest propagation loss in Formula (1), and the“MBest” means received power strength from the base station having thehighest received power strength in Formula (2).

“R1a” is a reporting range for defining occurrence conditions of a state(referred to as the event 1A in the 3GPP) in which a base station needsto be added as the active set cell, and “H1a” is a hysteresis value(value stored in the mobile station 10 to correct the reporting rangeR1a) of the event 1A.

“W” is a value transmitted from the base stations to the mobile station,and any of values (0.0 through 2.0) is assigned to the W.

Accordingly, when the broad propagation loss to the active set cell(W×10×Log(1/Σ(1/Mi))+(1−W)×10×Log Mbest+(R1a−H1a/2)) is compared withthe broad propagation loss to the monitor set cell (10×Log MNew+CIONew),in Formula (1), the W affects the value of the broad propagation lossfrom the active set cell.

Moreover, when the broad received power strength from the monitor setcell (10×Log MNew+CIONew) is compared with the broad received powerstrength from the active set cell (W×10×Log(ΣMi)+(1−W)×10×LogMbest−(R1a−H1a/2)), in Formula (2), the W affects the broad receivedpower strength from the active set cell.

Moreover, the event 1A judging unit 40 judges an occurrence of the event1A by calculating following Formula (3) based on propagation lossdifference “α” and received power difference “β” derived by the α/βderiving unit 34.α+β≧0  Formula (3)

Next, the operations of the mobile station 10 will be explainedaccording to FIG. 2 through FIG. 5.

In the mobile station 10 an event 1A judging program is periodicallyexecuted. When the event 1A judging program is executed, the α value isfirstly derived in the microcomputer 30 first derivation means), and theα value is stored into the α storage unit 36 (S2). The microcomputer 30calculates the β value after the α value has been derived (secondderivation means), and the β value is stored into the β storage unit 38(S4). The microcomputer 30 reads the α and β values from the α storageunit 36 and the β storage unit 38, respectively after the β value hasbeen derived, and executes a judging process by Formula (3) (S6)(comparing means). By this judging process, step S8 ensues when “α+β” issmaller than 0, or step S10 ensues when the α+β is greater than 0.

In step S8, a timer in the event 1A judging unit 40 is stopped when thetimer is in operation, or remains stopped when the timer is alreadystopped. Here, the timer is used in order to count the time that isreferred to as a “time-to-trigger” and defined in the 3GGP standard, andthe timer is used in order not to report to the base station unnecessaryinformation on the occurrence of the event 1A, even if the mobilestation 10 judges that the event 1A has occurred, if the judgment ismade instantaneously. In step S10, the microcomputer 30 judges whetherthe time-to-trigger period has terminated or not. As the result of thejudgment, when the timer is not yet started, the timer is started andthe event 1A process is terminated (S12), meanwhile, when the timer isalready started and the time-to-trigger period has elapsed, themicrocomputer 30 reports an occurrence of the event 1A to the basestation and then terminates the event 1A process (S14). Moreover, whenthe timer is already started but the time-to-trigger period has not yetelapsed, the microcomputer 30 terminates the event 1A judging processwithout doing anything.

Next, operations of the mobile station 10 in motion will be explainedaccording to FIG. 3 through FIG. 5. In FIG. 3, a base station “A” is anactive set cell, a base station “B” is a monitor set cell, and themobile station 10 moves in a direction “D” from a location near the basestation A toward near the base station B.

In FIG. 3 (a), a region “Ad” in which download signals can betransmitted from the base station A to the mobile station 10, isnarrower than a region “Au” in which upload signals can be transmittedfrom the mobile station 10 to the base station A. Moreover, a region“Bd” in which download signals can be transmitted from the base stationB to the mobile station 10, is wider than a region “Bu” in which uploadsignals can be transmitted from the mobile station 10 to the basestation B.

In this case, an upload-balance location “Pu” at which power levels ofthe upload signals transmitted from the mobile station 10 and receivedby the base stations A and B are balanced each other, may sometimesoccur after a download-balance location “Pd” at which power levels ofthe download signals transmitted from base stations A and B and receivedby the mobile station 10 are balanced each other, has occurred.Therefore, if a handover process is performed according to only theupload-balance location, when handover process is performed, their maybe a risk in that the position of the mobile station comes outsideregion in which the download signals can not be received from thecommunicating base stations A.

However, the mobile station 10 detects an occurrence of the event 1A ata halfway location between the upload-balance location Pu and thedownload-balance location Pd, and performs the handover process.Therefore, a possibility increases in which the handover process isperformed at a location where the download signals can be received fromthe communicating base station A.

On the other hand, in FIG. 3 (b), the region Ad in which the downloadsignals can be transmitted from the base station A to the mobile station10 is wider than the region Au in which the upload signals can betransmitted from the mobile station 10 to the base station A. Moreover,the region Bd in which the download signals can be transmitted from thebase station B to the mobile station 10 is narrower than the region Buin which the upload signals can be transmitted from the mobile station10 to the base station B.

In cases such as this, the download-balance location Pd may sometimesoccur after the upload-balance location has occurred.

However, because the mobile station 10 detects an occurrence of theevent 1A at a halfway location between the upload-balance location Puand the download-balance location Pd, and performs the handover process,a possibility increases in which the handover process is performed at alocation where the download signals can be received from thecommunicating base station A.

Moreover, when the mobile station 10 judges an occurrence of the event1A, operations of transmitting and receiving information will beexplained below according to FIG. 4, which are performed among themobile station 10, the base station as the active set cell (base stationA in FIG. 3), the base station as the monitor set cell (base station Bin FIG. 3), and a “RNC” (radio network control, master computer) forcontrolling each of the base stations.

When the mobile station 10 has judged the occurrence of the event 1A(judged that the base station B illustrated in FIG. 3 is added to theactive set cell) (S21), the mobile station 10 reports a signal, as a“measurement-report” (handover signal, handover request signal) by whichthe base station B is changed into the active set cell, to the RNCthrough a relaying means of the base station A (S23) (transmittingmeans). The RNC that has received the signal from the mobile station 10transmits an instruction signal, as a “radio-link-addition request”(handover request signal), to the base station B through an instructingmeans in the RNC, in order to change the communication mode with themobile station 10 into the active set cell (S25). The base station Bthat has received the signal changes the communication mode with themobile station 10 from the monitor set cell into the active set cell bya switching means (not illustrated) in the mobile station 10 (secondswitching means), and transmits to the RNC (S27) a response signal, a“radio-link-addition-response”, that means completion of the change. TheRNC that has received the response signal through a receiving means (notillustrated) in the RNC changes a setting state of the RNC into anotherstate in which the base station B is controlled as the active set cellof the mobile station 10 (S29). Moreover, at the same time as thischange, the RNC reports an “active-set-update” signal (handoverinstruction signal), which means that the base station B is changed intoan active set cell of the mobile station 10, to the mobile station 10through the relaying means of the base station A (S31). The mobilestation 10 receives this notification by a receiving means in the mobilestation 10, changes the base station B as a monitor set cell into thebase station as an active set cell by a switching means (notillustrated) in mobile station 10 (first switching means), and reportsan “active-set-update-complete” signal that means setting completion, tothe RNC through the base station A (S35).

Next, operations of detecting an occurrence of the event 1A at alocation “Ps” will be explained according to FIG. 5, where the locationPs is halfway between the location Pd at which received power levels ofdownload signals transmitted from the base stations A and B to themobile station 10 are balanced each other, and the location Pu at whichreceived power levels of upload signals transmitted from the mobilestation 10 to the base stations A and B are balanced each other.

In Formula (1), the a means the difference between the propagation lossto the base station A and the propagation loss to the base station B(the difference between the propagation loss to the first base stationand the propagation loss to the second base station). Moreover, aformula for calculating the difference between the propagation loss tothe base station A and the propagation loss to the base station B is notlimited to Formula (1), but it may be a formula capable of comparing thepropagation loss to the communicating base station A with thepropagation loss to the neighboring base station B.

Moreover, a propagation loss that rises during the time signalstransmitted from the base station reaches the mobile station isequivalent to a propagation loss that occurs during the time signalstransmitted from the mobile station reaches the base station. Therefore,the mobile station can judge, based on the propagation losses, whetherupload signals transmitted from the mobile station reaches the basestation or not. In other words, the fact that the propagation losses tothe base stations A and B are equivalent each other (α=0) means eachreceived power strength of upload signals received by the base stationsA and B is roughly equivalent.

Meanwhile, in Formula (2), the β means the difference between thereceived power strength from the base station A and the received powerstrength from the base station B (the difference between the receivedpower strength from the first base station and the received powerstrength from the second base station). Moreover, a formula forcalculating the difference between the received power strength from thebase station A and the received power strength from the base station Bis not limited to Formula (2), but it may be a formula capable ofcomparing the received power strength from the communicating basestation A with the received power strength from the neighboring basestation B.

Moreover, the mobile station can judge, based on the received powerstrength, whether download signals transmitted from the base stationreaches the mobile station or not. In other words, the fact that eachreceived power strength from the base stations A and B is equivalent(β=0) means each received power strength of the download signalstransmitted from the base stations A and B and received by the mobilestation is roughly equivalent.

FIG. 5 illustrates movements of the value α and the value β when themobile station is moving in the direction D illustrated in FIG. 3. InFIG. 5, both value α and value β trend upward. Moreover, if the timewhen the value α is 0 is different from the time when the value β is 0,one of the value α and the value β is positive and the other isnegative. Therefore, as illustrated in FIG. 5, there exists a timeperiod where Formula (3) is satisfied after the value β is firstlyvaried to a positive value, until the value α is varied to 0 next.

In other words, when Formula (3) is satisfied, the mobile station 10 ison the midway between a location at which the value β is varied to 0,and another location at which the value α is varied to 0. In otherwords, the mobile station 10 can detects an occurrence of the event 1Aand performs the handover process on the midway between a location atwhich the propagation losses to the base stations A and B are balancedeach other, and another location at which received power strengths fromthe base stations A and B are balanced each other.

Therefore, a possibility for the mobile station 10 to perform thehandover is higher at the location at which the download signals can bereceived from the base station A than at a location at which the uploadsignals to the base stations A and B are balanced each other.

Moreover, a possibility for the mobile station 10 to perform thehandover is higher at a location at which the upload signals can betransmitted to the base station A than at a location at which thedownload signals from the base stations A and B are balanced each other.

As described above, the mobile station 10 according to Embodiment 1 canperform the handover process without fail and provide a mobile stationthat has improved communication quality, even if a region in whichupload signals can reach a base station as an communicating active setcell is different from another region in which download signals can bereceived from the base station as the communicating active set cell.

Moreover, a communications system according to Embodiment 1 can performthe handover process without fail and improve communication quality,even if a region in which upload signals can reach a base station as thecommunicating active set cell, is different from another region in whichdownload signals can be received from the base station as thecommunicating active set cell.

Embodiment 2

Although the value α is simply added to the value β in Formula (3) inEmbodiment 1, the event 1A judging unit 40 in the mobile station 10 inEmbodiment 2 performs following Formula (4), in which the value α andthe value β each are multiplied by coefficients (correction values). Bycalculating Formula (4) as described above, judgment by Formula (3) iscorrected, enabling a judgment result to be more appropriate.cα+dβ≧0  Formula (4)

Moreover, if a storage device for storing the coefficients “c” and “d”is added to the mobile station 10 and the event 1A judging unit 40judges Formula (4) by reading predefined coefficients “c” and “d” fromthe storage device, more appropriate correction can be performed,because the correction values can be varied based on a specificcondition.

Embodiment 3

In Embodiment 3, a communications system as recited in Embodiment 1further includes: a storage device (not illustrated) for storing aplurality of coefficients “c” and “d” illustrated in Formula (4), and atransmitting means for transmitting signals to report to the basestation A (first base station) the relevant coefficients “c” and “d”read from the storage device, in the RNC; a relaying means (notillustrated), added to the base station A, for relaying to the mobilestation the information signals; and a receiving means, added to themobile station, for receiving the signals form the base station A.

In a communications system as described above, relevant coefficients “c”and “d” read from the storage device are transmitted from an RNC to abase station A (S41), information signals are relayed from the basestation A to a mobile station (S43), the information signals arereceived and stored in the mobile station, and the coefficients “c” and“d” can be used for a judgment using Formula (4).

Embodiment 4

In Embodiment 1, although a handover signal transmitted from the mobilestation 10 to the communicating base station is a signal (for example,an event 1A occurrence signal in the 3GGP standard) for adding a basestation as a monitor set cell to base stations as active set cells, themobile station 10 may transmit a signal (for example, an event 1Boccurrence signal in the 3GGP standard) for changing a base station asan active set cell into a base station as a monitor set cell, or asignal (for example, an event 1C occurrence signal in the 3GGP standard)for exchanging the mode of the first base station as a monitor set cellfor that of the second base station as an active set cell. In otherwords, the mobile station 10 may transmit a signal for performing thehandover process based on received power strength and propagation lossesof a communicating base station and a base station neighboring thecommunicating base station.

A communications system configured as described above can provide amobile station that executes the handover process without fail so as toimprove communication quality, even if a region in which upload signalscan reach the communicating base station is different from anotherregion in which download signals can be received from the communicatingbase station.

1. A mobile station, comprising: a first measuring means for measuringreceived power strength and propagation loss of signals transmitted froma communicating first base station; a second measuring means formeasuring received power strength and propagation loss of signalstransmitted from a second base station neighboring the first basestation; a first derivation means for deriving the difference betweenthe received power strengths from the first and second base stations, asmeasured by the first and second measuring means; a second derivationmeans for deriving the difference between the propagation losses fromthe first and second base stations, as measured by the first and secondmeasuring means; a comparing means for comparing the values derived bythe first and second derivation means; and a transmitting means fortransmitting, based on the comparison result by the comparing means, ahandover signal to the first base station.
 2. A mobile station asrecited in claim 1, wherein the comparing means adds a correction valueto the result derived by the first derivation means or the secondderivation means and outputs a comparison result.
 3. A mobile station asrecited in claim 2, further comprising a storage device for memorizing aplurality of correction values, wherein the comparing means outputs acomparison result corrected using any one of the correction valuesmemorized in the storage device.
 4. A mobile station as recited in claim2, further comprising a receiving means for receiving a signalrepresenting an arbitrary coefficient from a communicating base station,wherein the comparing means outputs a comparison result corrected usingthe received coefficient.
 5. A communications system, comprising: amobile station; a first base station communicating with the mobilestation; a second base station neighboring the first base station; and amaster computer for controlling communication with the first basestation and the second base station; wherein the mobile stationincludes: a first measuring means for receiving and measuring receivedpower strength and propagation loss of signals transmitted from thefirst base station; a second measuring means for receiving and measuringreceived power strength and propagation loss of signals transmitted fromthe second base station; a first derivation means for deriving thedifference between the received power strengths from the first andsecond base stations, as measured by the first and second measuringmeans; a second derivation means for deriving the difference between thepropagation losses from the first and second base stations, as measuredby the first and second measuring means; a comparing means for comparingthe values derived by the first and second derivation means; atransmitting means for transmitting, based on the comparison result bythe comparing means, to the first base station a handover request signalfor switching a communications channel to the second base station; and afirst switching means for establishing, based on a handover instructionsignal received from the first base station, the communications channelwith the second base station; the first base station includes a relayingmeans for relaying the handover request signal from the mobile stationto the master computer, and for relaying the handover instruction signalfrom the master computer to the mobile station; the master computerincludes a transmitting means for transmitting, based on the handoverrequest signal received from the mobile station, the handoverinstruction signal to the first and second base stations; and the secondbase station includes a second switching means for establishing, basedon the handover instruction signal received from the master computer,the communications channel with the mobile station.
 6. A communicationssystem as recited in claim 5, wherein the master computer furtherincludes; a storage means for memorizing a plurality of coefficients;and a transmitting means for transmitting, to the first base station, asignal representing a predefined coefficient read out from the storagemeans; wherein the first base station relays to the mobile station thesignal representing the predefined coefficient transmitted from themaster computer; and the comparing means of the mobile station outputs acomparison result corrected using the predefined coefficient receivedfrom the first base station.